Torpor Orb stops a creature’s own enters-the-battlefield triggered abilities as well as other triggered abilities that would trigger when a creature enters the battlefield.

6/1/2011

The trigger event doesn’t have to specify “creatures” entering the battlefield. For example, Amulet of Vigor says “Whenever a permanent enters the battlefield tapped and under your control, untap it.” If a creature enters the battlefield tapped and under your control, Amulet of Vigor would not trigger. If a land (that isn’t also a creature) enters the battlefield tapped and under your control, Amulet of Vigor would trigger.

6/1/2011

Abilities that create replacement effects, such as a permanent entering the battlefield tapped or with counters on it, are unaffected.

6/1/2011

Abilities that apply “as [this creature] enters the battlefield,” such as choosing a creature to copy with Clone, are unaffected.

6/1/2011

Look at the permanent as it exists on the battlefield, taking into account continuous effects, to determine whether any triggered abilities will trigger. For example, if you control March of the Machines, which says, in part, “Each noncreature artifact is an artifact creature,” Torpor Orb will be a creature when it enters the battlefield and will not cause triggered abilities to trigger.

6/1/2011

If Torpor Orb and a creature enter the battlefield at the same time, that creature entering the battlefield won’t cause triggered abilities to trigger.